Television receivers are commercially available which are capable of tuning either standard broadcast channels or cable television channels. The channel frequencies assigned to broadcast signals delivered "over-the-air" can differ from those used by cable service. For this reason so-called "cable ready" television receivers include a receiving mode control switch (or "AIR-CABLE" switch) which is manually settable to condition the receiver tuner to tune either broadcast or cable channels depending on whether the RF input is connected to a broadcast receiving antenna or to a cable TV distribution network. A further feature of certain cable-ready receivers is the inclusion of a channel memory which the user may program to store the channel numbers of particular channels he would like to receive and to delete the numbers of inactive or undesired channels. Receivers having this feature tune only to channels listed in the memory and skip over unlisted channels when in a channel scanning mode of operation (e.g., scan-UP or scan-DOWN) and so provide the user with a very rapid channel search.
A potential problem exists in cable-ready television receivers of the type described when switching between the cable and air operating modes. As an example, assume that the receiver is connected to a cable source and is tuned to cable channel 85. If the user then connects an antenna to the receiver for receiving broadcast signals and changes the cable/air switch to the air (broadcast) mode, the tuner will attempt to tune broadcast channel 85. There is, however, no channel 85 assigned to broadcast television services. In the absence of a "valid" channel 85 in the air mode, the receiver tuning voltage may be forced a maximum value, a situation which might be detrimental to the tuner and tuning system. Additionally, in this condition the user will be presented with visual noise instead of a picture and may experience some momentary confusion as to why the receiver will not operate in the air mode for the particular channel selected.
Heretofore this problem has been addressed by automatically setting the tuner to a specific channel (e.g., Channel 2) when the cable-air mode switch is changed if the previous channel tuned was not a "valid" channel in the new tuning mode. Certain cable-ready television receivers, such as those employing the model CTC-130 color television chassis manufactured by RCA Corporation, have such a feature.